Many semiconductor imaging sensors today are front side illuminated. That is, they include imaging arrays that are fabricated on the front side of a semiconductor wafer, where light is received at the imaging array from the same front side. However, front side illuminated imaging sensors have many drawbacks, one of which is a limited fill factor.
Backside illuminated imaging sensors are an alternative to front side illuminated imaging sensors that address the fill factor problems associated with front side illumination. Backside illuminated imaging sensors include imaging arrays that are fabricated on the front surface of the semiconductor wafer, but receive light through a back surface of the wafer. Color filters and micro-lenses may be included on the back surface of the wafer in order to improve the sensitivity of the backside illuminated sensor. However, to detect light from the backside, the wafer must be extremely thin. The thickness of the wafer may also be reduced in order to improve the sensitivity. Thus, the photodiode must be very close to the back surface of the sensor in order to generate and collect charge. However, infrared radiation is absorbed much deeper in the silicon than it is with visible light. A substantial amount of infrared light passes through the thinned silicon substrate and does not generate a related charge. Therefore, there is a tradeoff between visible light and infrared sensitivities with backside illuminated imaging sensors.